Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA (Wideband Code Division Multiple Access) radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In a longer time perspective it is, however, necessary to be prepared for further increasing user demands and an even tougher competition from new radio access technologies. To meet this challenge, 3GPP has initiated the study item Evolved UTRA and UTRAN (see 3GPP Tdoc. RP-040461, “Proposed Study Item on Evolved UTRA and UTRAN”, and 3GPP TR 25.912: “Feasibility study for evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN)”, version 7.2.0, June 2007, available at http://www.3gpp.org and both being incorporated herein by reference), aiming at studying means to achieve additional substantial leaps in terms of service provisioning and cost reduction. As a basis for this work, 3GPP has concluded on a set of targets and requirements for this long-term evolution (LTE) (see 3GPP TR 25.913, “Requirements for Evolved UTRA and Evolved UTRAN”, version 7.3.0, March 2006, available at http://www.3gpp.org, incorporated herein by reference) including for example:
Peak data rates exceeding 100 Mbps for the downlink direction and 50 Mbps for the uplink direction.
Mean user throughput improved by factors 2 and 3 for uplink and downlink respectively.
Cell-edge user throughput improved by a factor 2 for uplink and downlink.
Uplink and downlink spectrum efficiency improved by factors 2 and 3 respectively.
Significantly reduced control-plane latency.
Reduced cost for operator and end user.
Spectrum flexibility, enabling deployment in many different spectrum allocations.
The ability to provide high bit rates is a key measure for LTE. Multiple parallel data stream transmission to a single terminal, using multiple-input-multiple-output (MIMO) techniques, is one important component to reach this. Larger transmission bandwidth and at the same time flexible spectrum allocation are other pieces to consider when deciding what radio access technique to use. The choice of adaptive multi-layer OFDM, AML-OFDM, in downlink will not only facilitate to operate at different bandwidths in general but also large bandwidths for high data rates in particular. Varying spectrum allocations, ranging from 1.25 MHz to 20 MHz, are supported by allocating corresponding numbers of AML-OFDM subcarriers. Operation in both paired and unpaired spectrum is possible as both time-division and frequency-division duplex is supported by AML-OFDM.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of base stations (referred to as Node Bs or eNode Bs in the 3GPP terminology), providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (Radio Resource Control—RRC) protocol terminations towards the mobile terminal (referred to as UE in the 3GPP terminology).
The eNode B hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.
The eNode Bs are interconnected with each other by means of the X2 interface. The eNode Bs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNode Bs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNode B handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE. It manages and stores UE contexts, e.g., parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS).
The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the Sha interface towards the home HSS for roaming UEs.
OFDM With Frequency-Domain Adaptation
The AML-OFDM-based (AML-OFDM=Adaptive MultiLayer-Orthogonal Frequency Division Multiplex) downlink has a frequency structure based on a large number of individual sub-carriers with a spacing of 15 kHz. This frequency granularity facilitates to implement dual-mode UTRA/E-UTRA terminals. The ability to reach high bit rates is highly dependent on short delays in the system and a prerequisite for this is short sub-frame duration. Consequently, the LTE sub-frame duration is set as short as 1 ms in order to minimize the radio-interface latency. In order to handle different delay spreads and corresponding cell sizes with a modest overhead the OFDM cyclic prefix length can assume two different values. The shorter 4.7 ms cyclic prefix is enough to handle the delay spread for most unicast scenarios. With the longer cyclic prefix of 16.7 ms very large cells, up to and exceeding 120 km cell radius, with large amounts of time dispersion can be handled. In this case the length is extended by reducing the number of OFDM symbols in a sub-frame.
The basic principle of Orthogonal Frequency Division Multiplexing (OFDM) is to split the frequency band into a number of narrowband channels. Therefore, OFDM allows transmitting data on relatively flat parallel channels (subcarriers) even if the channel of the whole frequency band is frequency selective due to a multipath environment. Since the subcarriers experience different channel states, the capacities of the subcarriers vary and permit a transmission on each subcarrier with a distinct data-rate. Hence, subcarrier-wise (frequency domain) Link Adaptation (LA) by means of Adaptive Modulation and Coding (AMC) increases the radio efficiency by transmitting different data-rates over the subcarriers. OFDMA allows multiple users to transmit simultaneously on the different subcarriers per OFDM symbol. Since the probability that all users experience a deep fade in a particular subcarrier is very low, it can be assured that subcarriers are assigned to the users who see good channel gains on the corresponding sub-carriers. When allocating resources in the downlink to different users in a cell, the scheduler takes information on the channel status experienced by the users for the subcarriers into account. The control information signaled by the users, i.e., CQI, allows the scheduler to exploit the multi-user diversity, thereby increasing the spectral efficiency.
Localized vs. Distributed Mode
Two different resource allocation methods can be distinguished upon when considering a radio access scheme that distribute available frequency spectrum among different users as in OFDMA. The first allocation mode or “localized mode” tries to benefit fully from frequency scheduling gain by allocating the subcarriers on which a specific UE experiences the best radio channel conditions. Since this scheduling mode requires associated signaling (resource allocation signaling, CQI in uplink), this mode would be best suited for non-real time, high data rate oriented services. In the localized resource allocation mode a user is allocated continuous blocks of subcarriers.
The second resource allocation mode or “distributed mode” relies on the frequency diversity effect to achieve transmission robustness by allocating resources that are scattered over time and frequency grid. The fundamental difference with localized mode is that the resource allocation algorithm does not try to allocate the physical resources based on some knowledge on the reception quality at the receiver but select more or less randomly the resource it allocates to a particular UE. This distributed resource allocation method seems to be best suited for real-time services as less associated signaling (no fast CQI, no fast allocation signaling) relative to “localized mode” is required.
The two different resource allocation methods are shown in FIG. 3 and FIG. 4 for an OFDMA based radio access scheme. As can be seen from FIG. 3, which depicts the localized transmission mode, the localized mode is characterized by the transmitted signal having a continuous spectrum that occupies a part of the total available spectrum. Different symbol rates (corresponding to different data rates) of the transmitted signal imply different bandwidths (time/frequency bins) of a localized signal. On the other hand, as can be seen from FIG. 4, the distributed mode is characterized by the transmitted signal having a non-continuous spectrum that is distributed over more or less the entire system bandwidth (time/frequency bins).
Hybrid ARQ Schemes
A common technique for error detection and correction in packet transmission systems over unreliable channels is called hybrid Automatic Repeat request (HARM). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by a CRC (Cyclic Redundancy Check)), the receiver requests a retransmission of the packet. Generally (and throughout this document) the transmission of additional information is called “retransmission (of a packet)”, although this retransmission does not necessarily mean a transmission of the same encoded information, but could also mean the transmission of any information belonging to the packet (e.g., additional redundancy information).
Depending on the information (generally code-bits/symbols), of which the transmission is composed, and depending on how the receiver processes the information, the following Hybrid ARQ schemes are defined:
In Type I HARQ schemes, the information of the encoded packet is discarded and a retransmission is requested, if the receiver fails to decode a packet correctly. This implies that all transmissions are decoded separately. Generally, retransmissions contain identical information (code-bits/symbols) to the initial transmission.
In Type II HARQ schemes, a retransmission is requested, if the receiver fails to decode a packet correctly, where the receiver stores the information of the (erroneous received) encoded packet as soft information (soft-bits/symbols). This implies that a soft-buffer is required at the receiver. Retransmissions can be composed out of identical, partly identical or non-identical information (code-bits/symbols) according to the same packet as earlier transmissions. When receiving a retransmission the receiver combines the stored information from the soft-buffer and the currently received information and tries to decode the packet based on the combined information. (The receiver can also try to decode the transmission individually, however generally performance increases when combining transmissions.) The combining of transmissions refers to so-called soft-combining, where multiple received code-bits/symbols are likelihood combined and solely received code-bits/symbols are code combined. Common methods for soft-combining are Maximum Ratio Combining (MRC) of received modulation symbols and log-likelihood-ratio (LLR) combining (LLR combining only works for code-bits).
Type II schemes are more sophisticated than Type I schemes, since the probability for correct reception of a packet increases with receive retransmissions. This increase comes at the cost of a required hybrid ARQ soft-buffer at the receiver. This scheme can be used to perform dynamic link adaptation by controlling the amount of information to be retransmitted. E.g., if the receiver detects that decoding has been “almost” successful, it can request only a small piece of information for the next retransmission (smaller number of code-bits/symbols than in previous transmission) to be transmitted. In this case it might happen that it is even theoretically not possible to decode the packet correctly by only considering this retransmission by itself (non-self-decodable retransmissions).
Type III HARQ schemes may be considered a subset of Type II schemes: In addition to the requirements of a Type II scheme each transmission in a Type III scheme must be self-decodable.
HARQ Protocol Operation for Unicast Data Transmissions
A common technique for error detection and correction in packet transmission systems over unreliable channels is called hybrid Automatic Repeat request (HARQ). Hybrid ARQ is a combination of Forward Error Correction (FEC) and ARQ.
If a FEC encoded packet is transmitted and the receiver fails to decode the packet correctly (errors are usually checked by a CRC (Cyclic Redundancy Check)), the receiver requests a retransmission of the packet.
In LTE there are two levels of retransmissions for providing reliability, namely, HARQ at the MAC layer and outer ARQ at the RLC layer. The outer ARQ is required to handle residual errors that are not corrected by HARQ that is kept simple by the use of a single bit error-feedback mechanism, i.e., ACK/NACK. An N-process stop-and-wait HARQ is employed that has asynchronous retransmissions in the downlink and synchronous retransmissions in the uplink.
Synchronous HARQ means that the retransmissions of HARQ blocks occur at predefined periodic intervals. Hence, no explicit signaling is required to indicate to the receiver the retransmission schedule.
Asynchronous HARQ offers the flexibility of scheduling retransmissions based on air interface conditions. In this case some identification of the HARQ process needs to be signaled in order to allow for a correct combining and protocol operation. In 3GPP LTE systems, HARQ operations with eight processes is used. The HARQ protocol operation for downlink data transmission will be similar or even identical to HSDPA.
In uplink HARQ protocol operation there are two different options on how to schedule a retransmission. Retransmissions are either “scheduled” by a NACK (also referred to as a synchronous non-adaptive retransmission) or are explicitly scheduled by the network by transmitting a PDCCH (also referred to as synchronous adaptive retransmissions). In case of a synchronous non-adaptive retransmission the retransmission will use the same parameters as the previous uplink transmission, i.e., the retransmission will be signaled on the same physical channel resources, respectively uses the same modulation scheme/transport format.
Since synchronous adaptive retransmission are explicitly scheduled via PDCCH, the eNode B has the possibility to change certain parameters for the retransmission. A retransmission could be for example scheduled on a different frequency resource in order to avoid fragmentation in the uplink, or eNode B could change the modulation scheme or alternatively indicate to the user equipment what redundancy version to use for the retransmission. It should be noted that the HARQ feedback (ACK/NACK) and PDCCH signaling occurs at the same timing. Therefore the user equipment only needs to check once whether a synchronous non-adaptive retransmission is triggered (i.e., only a NACK is received) or whether eNode B requests a synchronous adaptive retransmission (i.e., PDCCH is signaled).
L1/L2 Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g., HARQ) L1/L2 control signaling is transmitted on the downlink along with the data. This control signaling is multiplexed with the downlink data in a sub-frame (assuming that the user allocation can change from sub-frame to sub-frame). Here, it should be noted, that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even be dynamic for each user. Generally, then the L1/2 control signaling needs only be transmitted once per TTI.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that assignments for uplink data transmissions, uplink (scheduling) grants, are also transmitted on the PDCCH.
Generally, the information sent on the L1/L2 control signaling may be separated into the two categories, Shared Control Information and Dedicated Control information:
Shared Control Information (SCI) Carrying Cat 1 Information
The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:
User identity, indicating the user which is allocated.
RB allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.
Duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.
Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Dedicated Control Information (DCI) Carrying Cat 2/3 Information
The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1.
The DCI typically contains information on:
Cat 2: Modulation scheme, transport-block (payload) size (or coding rate), MIMO related information, etc. (Note, either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated RBs)).
Cat 3: HARQ related information, e.g., hybrid ARQ process number, redundancy version, retransmission sequence number.
Details on L1/L2 Control Signaling Information
For downlink data transmissions L1/L2 control signaling is transmitted on a separate physical channel (PDCCH). This L1/L2 control signaling typically contains information on:
The physical resource(s) on which the data is transmitted (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.
The transport format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme may be signaled explicitly.
Hybrid ARQ (HARD) Information:
Process number: Allows the user equipment to identify the Hybrid ARQ process on which the data is mapped.
Sequence number or new data indicator: Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet.
Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).
UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:
The physical resource(s) on which the user equipment should transmit the data (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).
The transport Format, the UE should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.
Hybrid ARQ Information:
Process number: Tells the user equipment from which Hybrid ARQ process it should pick the data.
Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet.
Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).
UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. E.g.:
HARQ process number may not be needed in case of a synchronous HARQ protocol.
A redundancy and/or constellation version may not be needed if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is predefined.
Power control information may be additionally included in the control signaling.
MIMO related control information, such as, e.g., precoding, may be additionally included in the control signaling.
In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.
For uplink resource assignments (for the Physical Uplink Shared Channel—PUSCH) signaled on the PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e., the redundancy version information is embedded in the transport format (TF) field. The TF field respectively MCS field (Modulation and Coding Scheme field) has for example a size of 5 bits, which corresponds to 32 indices. Three TF/MCS table indices are reserved for indicating RVs 1, 2 or 3. The remaining MCS table indices are used to signal the MCS level (transport block size—TBS) implicitly indicating RV0. The TBS/RV signaling for uplink assignments on PDCCH is shown in Table 1 below. An exemplary PDCCH for uplink resource assignments is shown in FIG. 5. The fields FH (Frequency Hopping), Cyclic shift and CQI (Channel Quality Index) are physical layer parameters and of no specific importance for understanding the invention described herein, so that their description is omitted. The size of the CRC field of the PDCCH is 16 bits. For further, more detailed information on the information fields contained in a PDCCH for uplink resource assignments, e.g., DCI format 0, it is referred to section 5.3.3.1 of 3GPP TS 36.212“Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”, version 8.3.0, June 2008, available at http://www.3gpp.org and the entire document being incorporated herein by reference. Even though the field providing transport format respectively modulation and coding scheme and redundancy version information is referred to as “modulation and coding scheme and redundancy version” it will be for the further description of the invention only referred to as modulation and coding scheme (MCS) field.
For downlink resource assignments (for the Physical Downlink Shared Channel—PDSCH) signaled on PDCCH in LTE the redundancy version is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information, similar to the uplink case there is 5 bit MCS field signaled on PDCCH. Three of the indices are reserved for the signaling of an explicit modulation order, i.e., those indices do not provide any transport format (transport block size) information. The remaining 29 indices signal modulation order and transport block size information as shown in Table 3 below. For further, more detailed information on the PDCCH formats for downlink resource assignment it is again referred to section 5.3.3.1 of 3GPP TS 36.212. For example, section 5.3.3.1.3 describes the DCI format 1A, which is one of the DCI formats for scheduling PDSCH. For downlink assignments the field providing transport block size and modulation order information is referred to as “modulation and coding scheme” field the term that will also be used in the description of this invention.
UL/DL Grant Reception Behavior
Generally the grant reception procedure (i.e., the procedure of receiving a resource assignment) is split between Physical layer and MAC layer. The Physical layer detects an uplink/downlink resource assignment on the PDCCH, extracts and determines certain information from the PDCCH fields and reports this to MAC layer. The MAC layer is responsible for the protocol procedures, i.e., HARQ protocol operation for uplink/downlink transmissions. Also the scheduling procedures for dynamic as well as semi-persistent scheduling are handled within the MAC layer.
When receiving a resource assignment on the PDCCH for uplink respectively downlink, the physical layer needs to determine certain information from received PDCCH fields which are required for the further processing of the assignments in MAC layer. As described in 3GPP TS 36.213, the Physical layer needs to determine the modulation order and transport block size in the PDSCH for a downlink resource assignment. The calculation of modulation order and transport block size is described in section 7.1.7 of 3GPP TS 36.213. Transport block size together with the HARQ process ID and the NDI bit is delivered to the MAC layer, which requires this information for performing the downlink HARQ protocol operation. The information delivered from Physical layer (Layer 1) to MAC (Layer 2) is also referred to as HARQ information.
Similar to the downlink, the Physical layer calculates modulation order and transport block size from received PDCCH containing the uplink resource assignment as described in section 8.6 of 3GPP TS 36.213. The Physical layer reports the calculated transport block size, redundancy version (RV) as well as NDI information of the PDCCH within the HARQ information to the MAC layer.
Semi-Persistent Scheduling (SPS)
In the downlink and uplink, the scheduling eNode B dynamically allocates resources to user equipments at each transmission time interval via the L1/L2 control channel(s) (PDCCH) where the user equipments are addressed via their specific C-RNTIs. As already mentioned before the CRC of a PDCCH is masked with the addressed user equipment's C-RNTI (so-called dynamic PDCCH). Only a user equipment with a matching C-RNTI can decode the PDCCH content correctly, i.e., the CRC check is positive. This kind of PDCCH signaling is also referred to as dynamic (scheduling) grant. A user equipment monitors at each transmission time interval the L1/L2 control channel(s) for a dynamic grant in order to find a possible allocation (downlink and uplink) it is assigned to.
In addition, E-UTRAN can allocate uplink/downlink resources for initial HARQ transmissions persistently. When required, retransmissions are explicitly signaled via the L1/L2 control channel(s). Since retransmissions are scheduled, this kind of operation is referred to as semi-persistent scheduling (SPS), i.e., resources are allocated to the user equipment on a semi-persistent basis (semi-persistent resource allocation). The benefit is that PDCCH resources for initial HARQ transmissions are saved. For details on semi-persistent scheduling, see 3GPP TS 36.300, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)”, version 8.5.0, June 2008 or 3GPP TS 36.321“Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification (Release 8)”, version 8.2.0, June 2008, both available at http://www.3gpp.org and incorporated herein by reference.
One example for a service, which might be scheduled using semi-persistent scheduling is Voice over IP (VoiP). Every 20 ms a VoIP packet is generated at the codec during a talk-spurt. Therefore eNode B could allocated uplink or respectively downlink resource persistently every 20 ms, which could be then used for the transmission of Voice over IP packets. In general, semi-persistent scheduling is beneficial for services with a predictable traffic behavior, i.e., constant bit rate, packet arrival time is periodic.
The user equipment also monitors the PDCCHs in a sub-frame where it has been allocated resources for an initial transmission persistently. A dynamic (scheduling) grant, i.e., PDCCH with a C-RNTI-masked CRC, can override a semi-persistent resource allocation. In case the user equipment finds its C-RNTI on the L1/L2 control channel(s) in the sub-frames where the sub-frame has a semi-persistent resource assigned, this L1/L2 control channel allocation overrides the semi-persistent resource allocation for that transmission time interval and the user equipment does follow the dynamic grant. When sub-frame does not find a dynamic grant it will transmit/receive according to the semi-persistent resource allocation.
The configuration of semi-persistent scheduling is done by RRC signaling. For example the periodicity, i.e., PS_PERIOD, of the persistent allocation is signaled within Radio resource Control (RRC) signaling. The activation of a persistent allocation and also the exact timing as well as the physical resources and transport format parameters are sent via PDCCH signaling. Once semi-persistent scheduling is activated, the user equipment follows the semi-persistent resource allocation according to the activation SPS PDCCH every PS_PERIOD. Essentially the user equipment stores the SPS activation PDCCH content and follows the PDCCH with the signaled periodicity.
In order to distinguish a dynamic PDCCH from a PDCCH, which activates semi-persistent scheduling, i.e., also referred to as SPS activation PDCCH, a separate identity is introduced. Basically, the CRC of a SPS activation PDCCH is masked with this additional identity which is in the following referred to as SPS C-RNTI. The size of the SPS C-RNTI is also 16 bits, same as the normal C-RNTI. Furthermore the SPS C-RNTI is also user equipment-specific, i.e., each user equipment configured for semi-persistent scheduling is allocated a unique SPS C-RNTI.
In case a user equipment detects a semi-persistent resource allocation is activated by a corresponding SPS PDCCH, the user equipment will store the PDCCH content (i.e., the semi-persistent resource assignment) and apply it every semi-persistent scheduling interval, i.e., periodicity signaled via RRC. As already mentioned, a dynamic allocation, i.e., signaled on dynamic PDCCH, is only a “one-time allocation”.
Similar to the activation of semi-persistent scheduling, the eNode B also can deactivate semi-persistent scheduling. There are several options how a semi-persistent scheduling de-allocation can be signaled. One option would be to use PDCCH signaling, i.e., SPS PDCCH indicating a zero size resource allocation, another option would be to use MAC control signaling.
Reduction of SPS False Activation
When the user equipment monitors the PDCCH for assignments, there is always a certain probability (false alarm rate), that the user equipment falsely considers a PDCCH destined to itself. Essentially, situations may occur where the CRC check of the PDCCH is correct even though the PDCCH was not intended for this user equipment, i.e., CRC passes even though there is a UE identifier (UE ID) mismatch (unintended user). These so-called “false alarms” might occur, if the two effects of transmission errors caused by the radio channel and UE ID mismatch cancel each other. The probability of a falsely positive decoded PDCCH depends on the CRC length. The longer the CRC length, the lower the probability that a CRC-protected message is erroneously decoded correctly. With the CRC size of 16 bit the false alarm probability would be 1.5e-05. It should be noted that due to the introduction of a separate identity for the discrimination of dynamic PDCCHs (dynamic C-RNTI) and SPS PDCCHs (SPS C-RNTI), false alarms are even more frequent.
On the first glance the probability might appear to be sufficiently low, however the impacts of a falsely positive decoded semi-persistent scheduling PDCCH are quite severe as will be outlined in the following. Since the effects are in particular for uplink persistent allocation critical, the main focus lies on falsely activated uplink semi-persistent resource allocations.
In case the UE falsely detects a SPS UL PDCCH (i.e., an uplink resource assignment for a semi-persistent resource allocation), the content of the PDCCH is some random value. Consequently UE transmits on PUSCH using some random RB location and bandwidth found in the false positive grant, which subjects the eNode B to UL interferences. With 50% probability UE jams more than half the bandwidth of the system since the Resource Allocation field is random. The user equipment is looking for ACK/NACK in the location corresponding to the (false positive) semi-persistent uplink resource allocation. The eNode B is not transmitting any data to the user equipment and the user equipment will decode the “acknowledgment” for its transmission (ACK/NACK) pretty random. When a NACK is received user equipment performs a synchronous non-adaptive retransmission. When ACK is received user equipment is suspended until the next SPS occasion, and the MAC may assume the transport block has been successfully received and decoded at the eNode B.
Essentially as a consequence of a false activation of a semi-persistent resource allocation for the uplink, a talk spurt can be lost completely or partially several times during a normal voice call. In addition, a false activation of a semi-persistent resource allocation for the uplink causes unnecessary interference to the system.
Given the severe consequences it is desirable to significantly increase the average time of false semi-persistent scheduling activations. One means to lower the false alarm rate to an acceptable level is to use a “Virtual CRC” in order to expand the 16-bit CRC: The length of the CRC field can be virtually extended by setting fixed and known values to some of the PDCCH fields that are not useful for semi-persistent scheduling activation. The user equipment shall ignore the PDCCH for semi-persistent resource activation if the values in these fields are not correct. Since MIMO operation with semi-persistent scheduling does not seem to be that useful, the corresponding PDCCH fields could be used in order to the increase the virtual CRC length. One further example is the NDI field. As already mentioned the NDI bit should be set to 0 on a PDCCH for semi-persistent scheduling activation. The false alarm rate could be further reduced by restricting the set of transport block sizes, which are valid for a semi-persistent scheduling activation.
As mentioned above, a semi-persistent scheduling resource release is signaled by means of an PDCCH similar to an SPS activation. In order to use the resource for SPS efficiently, it is desirable that resources can be re-allocated quickly, for example in VoIP by means of explicit release of a persistent allocation during silence periods in speech, followed by a re-activation when the silence periods end. Therefore it should be noted that at a semi-persistent scheduling resource release an SPS RRC configuration, e.g., PS_PERIOD, remains in place until changed by RRC signaling. Therefore PDCCH is used for an efficient explicit release (de-activation) of semi-persistent scheduling.
One possibility would be sending a semi-persistent scheduling activation with a zero-size resource allocation. A zero-size allocation would correspond to a resource allocation of 0 physical resource blocks (RB) which would effectively deactivate the semi-persistent resource allocation. This solution requires that a PDCCH message, i.e., uplink/downlink resource assignment, is able to indicate “ORBs” as one possible resource block allocation. Since this is not possible with the PDCCH formats agreed on in the 3GPP, a new “ORB” entry would need to be introduced in resource block assignment field for PDSCH and PUSCH. This would however also have impact on the Physical layer-to-MAC Layer interaction in the user equipments, as the Physical layer would further need to be adapted to inform the MAC layer on the deactivation of the semi-persistent resource allocation.